src/include/gpxe/tables.h

   1 #ifndef _GPXE_TABLES_H
   2 #define _GPXE_TABLES_H
   3
   4 /** @page ifdef_harmful #ifdef considered harmful
   5  *
   6  * Overuse of @c #ifdef has long been a problem in Etherboot.
   7  * Etherboot provides a rich array of features, but all these features
   8  * take up valuable space in a ROM image.  The traditional solution to
   9  * this problem has been for each feature to have its own @c #ifdef
  10  * option, allowing the feature to be compiled in only if desired.
  11  *
  12  * The problem with this is that it becomes impossible to compile, let
  13  * alone test, all possible versions of Etherboot.  Code that is not
  14  * typically used tends to suffer from bit-rot over time.  It becomes
  15  * extremely difficult to predict which combinations of compile-time
  16  * options will result in code that can even compile and link
  17  * correctly.
  18  *
  19  * To solve this problem, we have adopted a new approach from
  20  * Etherboot 5.5 onwards.  @c #ifdef is now "considered harmful", and
  21  * its use should be minimised.  Separate features should be
  22  * implemented in separate @c .c files, and should \b always be
  23  * compiled (i.e. they should \b not be guarded with a @c #ifdef @c
  24  * MY_PET_FEATURE statement).  By making (almost) all code always
  25  * compile, we avoid the problem of bit-rot in rarely-used code.
  26  *
  27  * The file config.h, in combination with the @c make command line,
  28  * specifies the objects that will be included in any particular build
  29  * of Etherboot.  For example, suppose that config.h includes the line
  30  *
  31  * @code
  32  *
  33  *   #define CONSOLE_SERIAL
  34  *   #define DOWNLOAD_PROTO_TFTP
  35  *
  36  * @endcode
  37  *
  38  * When a particular Etherboot image (e.g. @c bin/rtl8139.zdsk) is
  39  * built, the options specified in config.h are used to drag in the
  40  * relevant objects at link-time.  For the above example, serial.o and
  41  * tftp.o would be linked in.
  42  *
  43  * There remains one problem to solve: how do these objects get used?
  44  * Traditionally, we had code such as
  45  *
  46  * @code
  47  *
  48  *    #ifdef CONSOLE_SERIAL
  49  *      serial_init();
  50  *    #endif
  51  *
  52  * @endcode
  53  *
  54  * in main.c, but this reintroduces @c #ifdef and so is a Bad Idea.
  55  * We cannot simply remove the @c #ifdef and make it
  56  *
  57  * @code
  58  *
  59  *   serial_init();
  60  *
  61  * @endcode
  62  *
  63  * because then serial.o would end up always being linked in.
  64  *
  65  * The solution is to use @link tables.h linker tables @endlink.
  66  *
  67  */
  68
  69 /** @file
  70  *
  71  * Linker tables
  72  *
  73  * Read @ref ifdef_harmful first for some background on the motivation
  74  * for using linker tables.
  75  *
  76  * This file provides macros for dealing with linker-generated tables
  77  * of fixed-size symbols.  We make fairly extensive use of these in
  78  * order to avoid @c #ifdef spaghetti and/or linker symbol pollution.
  79  * For example, instead of having code such as
  80  *
  81  * @code
  82  *
  83  *    #ifdef CONSOLE_SERIAL
  84  *      serial_init();
  85  *    #endif
  86  *
  87  * @endcode
  88  *
  89  * we make serial.c generate an entry in the initialisation function
  90  * table, and then have a function call_init_fns() that simply calls
  91  * all functions present in this table.  If and only if serial.o gets
  92  * linked in, then its initialisation function will be called.  We
  93  * avoid linker symbol pollution (i.e. always dragging in serial.o
  94  * just because of a call to serial_init()) and we also avoid @c
  95  * #ifdef spaghetti (having to conditionalise every reference to
  96  * functions in serial.c).
  97  *
  98  * The linker script takes care of assembling the tables for us.  All
  99  * our table sections have names of the format @c .tbl.NAME.NN where
 100  * @c NAME designates the data structure stored in the table (e.g. @c
 101  * init_fn) and @c NN is a two-digit decimal number used to impose an
 102  * ordering upon the tables if required.  @c NN=00 is reserved for the
 103  * symbol indicating "table start", and @c NN=99 is reserved for the
 104  * symbol indicating "table end".
 105  *
 106  * As an example, suppose that we want to create a "frobnicator"
 107  * feature framework, and allow for several independent modules to
 108  * provide frobnicating services.  Then we would create a frob.h
 109  * header file containing e.g.
 110  *
 111  * @code
 112  *
 113  *   struct frobnicator {
 114  *      const char *name;               // Name of the frobnicator
 115  *      void ( *frob ) ( void );        // The frobnicating function itself
 116  *   };
 117  *
 118  *   #define __frobnicator __table ( frobnicators, 01 )
 119  *
 120  * @endcode
 121  *
 122  * Any module providing frobnicating services would look something
 123  * like
 124  *
 125  * @code
 126  *
 127  *   #include "frob.h"
 128  *
 129  *   static void my_frob ( void ) {
 130  *      // Do my frobnicating
 131  *      ...
 132  *   }
 133  *
 134  *   struct frob my_frobnicator __frobnicator = {
 135  *      .name = "my_frob",
 136  *      .frob = my_frob,
 137  *   };
 138  *
 139  * @endcode
 140  *
 141  * The central frobnicator code (frob.c) would use the frobnicating
 142  * modules as follows
 143  *
 144  * @code
 145  *
 146  *   #include "frob.h"
 147  *
 148  *   static struct frob frob_start[0] __table_start ( frobnicators );
 149  *   static struct frob frob_end[0] __table_end ( frobnicators );
 150  *
 151  *   // Call all linked-in frobnicators
 152  *   void frob_all ( void ) {
 153  *      struct frob *frob;
 154  *
 155  *      for ( frob = frob_start ; frob < frob_end ; frob++ ) {
 156  *         printf ( "Calling frobnicator \"%s\"\n", frob->name );
 157  *         frob->frob ();
 158  *      }
 159  *   }
 160  *
 161  * @endcode
 162  *
 163  * See init.h and init.c for a real-life example.
 164  *
 165  */
 166
 167 #ifdef DOXYGEN
 168 #define __attribute__( x )
 169 #endif
 170
 171 #define __table_str( x ) #x
 172 #define __table_section( table, idx ) \
 173         __section__ ( ".tbl." __table_str ( table ) "." __table_str ( idx ) )
 174
 175 #define __table_section_start( table ) __table_section ( table, 00 )
 176 #define __table_section_end( table ) __table_section ( table, 99 )
 177
 178 #define __natural_alignment( type ) __aligned__ ( __alignof__ ( type ) )
 179
 180 /**
 181  * Linker table entry.
 182  *
 183  * Declares a data structure to be part of a linker table.  Use as
 184  * e.g.
 185  *
 186  * @code
 187  *
 188  *   struct my_foo __table ( foo, 01 ) = {
 189  *      ...
 190  *   };
 191  *
 192  * @endcode
 193  *
 194  */
 195 #define __table( type, table, idx )                             \
 196         __attribute__ (( __table_section ( table, idx ),        \
 197                          __natural_alignment ( type ) ))
 198
 199 /**
 200  * Linker table start marker.
 201  *
 202  * Declares a data structure (usually an empty data structure) to be
 203  * the start of a linker table.  Use as e.g.
 204  *
 205  * @code
 206  *
 207  *   static struct foo_start[0] __table_start ( foo );
 208  *
 209  * @endcode
 210  *
 211  */
 212 #define __table_start( type, table ) __table ( type, table, 00 )
 213
 214 /**
 215  * Linker table end marker.
 216  *
 217  * Declares a data structure (usually an empty data structure) to be
 218  * the end of a linker table.  Use as e.g.
 219  *
 220  * @code
 221  *
 222  *   static struct foo_end[0] __table_end ( foo );
 223  *
 224  * @endcode
 225  *
 226  */
 227 #define __table_end( type, table ) __table ( type, table, 99 )
 228
 229 #endif /* _GPXE_TABLES_H */